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<body class="article">
<div id="header">
<h1>Keep authoritative canonical history correct with git pull</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>Sometimes a new project integrator will end up with project history
that appears to be "backwards" from what other project developers
expect. This howto presents a suggested integration workflow for
maintaining a central repository.</p></div>
<div class="paragraph"><p>Suppose that that central repository has this history:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    ---o---o---A</code></pre>
</div></div>
<div class="paragraph"><p>which ends at commit <code>A</code> (time flows from left to right and each node
in the graph is a commit, lines between them indicating parent-child
relationship).</p></div>
<div class="paragraph"><p>Then you clone it and work on your own commits, which leads you to
have this history in <strong>your</strong> repository:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    ---o---o---A---B---C</code></pre>
</div></div>
<div class="paragraph"><p>Imagine your coworker did the same and built on top of <code>A</code> in <strong>his</strong>
repository in the meantime, and then pushed it to the
central repository:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    ---o---o---A---X---Y---Z</code></pre>
</div></div>
<div class="paragraph"><p>Now, if you <code>git push</code> at this point, because your history that leads
to <code>C</code> lacks <code>X</code>, <code>Y</code> and <code>Z</code>, it will fail.  You need to somehow make
the tip of your history a descendant of <code>Z</code>.</p></div>
<div class="paragraph"><p>One suggested way to solve the problem is "fetch and then merge", aka
<code>git pull</code>. When you fetch, your repository will have a history like
this:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    ---o---o---A---B---C
                \
                 X---Y---Z</code></pre>
</div></div>
<div class="paragraph"><p>Once you run merge after that, while still on <strong>your</strong> branch, i.e. <code>C</code>,
you will create a merge <code>M</code> and make the history look like this:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    ---o---o---A---B---C---M
                \         /
                 X---Y---Z</code></pre>
</div></div>
<div class="paragraph"><p><code>M</code> is a descendant of <code>Z</code>, so you can push to update the central
repository.  Such a merge <code>M</code> does not lose any commit in both
histories, so in that sense it may not be wrong, but when people want
to talk about "the authoritative canonical history that is shared
among the project participants", i.e. "the trunk", they often view
it as "commits you see by following the first-parent chain", and use
this command to view it:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    $ git log --first-parent</code></pre>
</div></div>
<div class="paragraph"><p>For all other people who observed the central repository after your
coworker pushed <code>Z</code> but before you pushed <code>M</code>, the commit on the trunk
used to be <code>o-o-A-X-Y-Z</code>.  But because you made <code>M</code> while you were on
<code>C</code>, <code>M</code>'s first parent is <code>C</code>, so by pushing <code>M</code> to advance the
central repository, you made <code>X-Y-Z</code> a side branch, not on the trunk.</p></div>
<div class="paragraph"><p>You would rather want to have a history of this shape:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    ---o---o---A---X---Y---Z---M'
                \             /
                 B-----------C</code></pre>
</div></div>
<div class="paragraph"><p>so that in the first-parent chain, it is clear that the project first
did <code>X</code> and then <code>Y</code> and then <code>Z</code> and merged a change that consists of
two commits <code>B</code> and <code>C</code> that achieves a single goal.  You may have
worked on fixing the bug #12345 with these two patches, and the merge
<code>M'</code> with swapped parents can say in its log message "Merge
fix-bug-12345". Having a way to tell <code>git pull</code> to create a merge
but record the parents in reverse order may be a way to do so.</p></div>
<div class="paragraph"><p>Note that I said "achieves a single goal" above, because this is
important.  "Swapping the merge order" only covers a special case
where the project does not care too much about having unrelated
things done on a single merge but cares a lot about first-parent
chain.</p></div>
<div class="paragraph"><p>There are multiple schools of thought about the "trunk" management.</p></div>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
Some projects want to keep a completely linear history without any
    merges.  Obviously, swapping the merge order would not match their
    taste.  You would need to flatten your history on top of the
    updated upstream to result in a history of this shape instead:
</p>
<div class="listingblock">
<div class="content">
<pre><code>    ---o---o---A---X---Y---Z---B---C</code></pre>
</div></div>
<div class="paragraph"><p>with <code>git pull --rebase</code> or something.</p></div>
</li>
<li>
<p>
Some projects tolerate merges in their history, but do not worry
    too much about the first-parent order, and allow fast-forward
    merges.  To them, swapping the merge order does not hurt, but
    it is unnecessary.
</p>
</li>
<li>
<p>
Some projects want each commit on the "trunk" to do one single
    thing.  The output of <code>git log --first-parent</code> in such a project
    would show either a merge of a side branch that completes a single
    theme, or a single commit that completes a single theme by itself.
    If your two commits <code>B</code> and <code>C</code> (or they may even be two groups of
    commits) were solving two independent issues, then the merge <code>M'</code>
    we made in the earlier example by swapping the merge order is
    still not up to the project standard.  It merges two unrelated
    efforts <code>B</code> and <code>C</code> at the same time.
</p>
</li>
</ol></div>
<div class="paragraph"><p>For projects in the last category (Git itself is one of them),
individual developers would want to prepare a history more like
this:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>                 C0--C1--C2     topic-c
                /
    ---o---o---A                master
                \
                 B0--B1--B2     topic-b</code></pre>
</div></div>
<div class="paragraph"><p>That is, keeping separate topics on separate branches, perhaps like
so:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    $ git clone $URL work &amp;&amp; cd work
    $ git checkout -b topic-b master
    $ ... work to create B0, B1 and B2 to complete one theme
    $ git checkout -b topic-c master
    $ ... same for the theme of topic-c</code></pre>
</div></div>
<div class="paragraph"><p>And then</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    $ git checkout master
    $ git pull --ff-only</code></pre>
</div></div>
<div class="paragraph"><p>would grab <code>X</code>, <code>Y</code> and <code>Z</code> from the upstream and advance your master
branch:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>                 C0--C1--C2     topic-c
                /
    ---o---o---A---X---Y---Z    master
                \
                 B0--B1--B2     topic-b</code></pre>
</div></div>
<div class="paragraph"><p>And then you would merge these two branches separately:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    $ git merge topic-b
    $ git merge topic-c</code></pre>
</div></div>
<div class="paragraph"><p>to result in</p></div>
<div class="listingblock">
<div class="content">
<pre><code>                 C0--C1---------C2
                /                 \
    ---o---o---A---X---Y---Z---M---N
                \             /
                 B0--B1-----B2</code></pre>
</div></div>
<div class="paragraph"><p>and push it back to the central repository.</p></div>
<div class="paragraph"><p>It is very much possible that while you are merging topic-b and
topic-c, somebody again advanced the history in the central repository
to put <code>W</code> on top of <code>Z</code>, and make your <code>git push</code> fail.</p></div>
<div class="paragraph"><p>In such a case, you would rewind to discard <code>M</code> and <code>N</code>, update the
tip of your <em>master</em> again and redo the two merges:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>    $ git reset --hard origin/master
    $ git pull --ff-only
    $ git merge topic-b
    $ git merge topic-c</code></pre>
</div></div>
<div class="paragraph"><p>The procedure will result in a history that looks like this:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>                 C0--C1--------------C2
                /                     \
    ---o---o---A---X---Y---Z---W---M'--N'
                \                 /
                 B0--B1---------B2</code></pre>
</div></div>
<div class="paragraph"><p>See also <a href="http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html">http://git-blame.blogspot.com/2013/09/fun-with-first-parent-history.html</a></p></div>
</div>
</div>
</div>
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Last updated 2015-09-28 15:52:41 PDT
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